vim /etc/gitlab/gitlab.rb
## GitLab configuration settings
##! This file is generated during initial installation and **is not** modified
##! during upgrades.
##! Check out the latest version of this file to know about the different
##! settings that can be configured by this file, which may be found at:
##! https://gitlab.com/gitlab-org/omnibus-gitlab/raw/master/files/gitlab-config-template/gitlab.rb.template
## GitLab URL
##! URL on which GitLab will be reachable.
##! For more details on configuring external_url see:
##! https://docs.gitlab.com/omnibus/settings/configuration.html#configuring-the-external-url-for-gitlab
external_url 'http://your_domain:8888'
...